Radicals acyloxy alkyl

Peroxyesters undergo single- or multiple-bond scission to generate acyloxy and alkoxy radicals, or alkyl and alkoxy radicals and carbon dioxide ... 

In parallel with the development of the heterolysis of b-substituted alkyl radicals, a rearrangement reaction was observed and extensively studied in organic solvents. This rearrangement was first noted for b-(acyloxy)alkyl radicals (Scheme 5) by Surzur et al. [48] and, later, for b-(phosphatoxy)alkyl radicals by the Crich and Giese groups [49,50]. 

The 1,2 migration of an acyloxy group in (3-(acyloxy)alkyl radicals has been the subject of numerous synthetic and mechanistic studies [25]. These... 

In this context, it is worth mentioning that the C(l )-substituted C(2 ) radical undergoes rapid P-(acyloxy) alkyl rearrangement [reaction (261)], whereby a C(l )-type radical is also formed (Gimisis et al. 1995,1998). 

The use of hypervalent iodine reagents in carbon-carbon bond forming reactions is summarized with particular emphasis on applications in organic synthesis. The most important recent methods involve the radical decarboxylative alkylation of organic substrates with [bis(acyloxy)iodo]arenes, spirocyclization of para- and ortho-substituted phenols, the intramolecular oxidative coupling of phenol ethers, and the reactions of iodonium salts and ylides. A significant recent research activity is centered in the area of the transition metal-mediated coupling reactions of the alkenyl-, aryl-, and alkynyliodonium salts. 

Radical Decarboxylative Alkylation with [Bis(acyloxy)iodo]arenes 101... 

Radical decarboxylative alkylation of heteroaromatic bases mediated by [bis(acyloxy)iodo]arenes... 

To date it has not been possible to observe directly radical cations in any of the j8-(acyloxy)alkyl migrations studied by the time-resolved laser flash photolysis method... 

Extensive computational studies have been carried out on the )8-(acyloxy)alkyl and ff-( phosphatoxy)alkyl rearrangements by Radom and coworkers and by Zipse. These calculations in general support the possibility of concerted rearrangements taking place via 5-center-5-electron and 3-center-3-electron cyclic transition states [25, 26]. However, before such computations can be used as an aid in distinguishing between reaction pathways, it will be necessary for theoretical chemists to circumvent the present difficulties in calculating the radical ionic fragmentations. 

The chemistry of )9-(thiocarbonyloxy)alkyl radicals stands in complete contrast to that of the (acyloxy)alkyl radicals, with elimination, while not the rule, being the norm [I]. The difference between the acyloxy and thiocarbonyloxy series is likely a consequence of the much weaker thiocarbonyl bond and the related higher stability of sulfur-centered radicals. The method has been developed in combination with the Barton deoxygenation method (Volume 1, Chapter 1.6) as a means of converting a vicinal diol, via the dixanthate, into an alkene (Scheme 33) [60-62]. Tributyltin hydride has been the reagent of choice for this reaction but it may also be conducted with the triethylsilane/benzoyl peroxide couple [63] and, doubtless, tris(trimethylsilyl)silane. 

Beckwith, A. L. J., Crich, D., Duggan, P. J., and Yao, Q. W., Chemistry of beta-(acyloxy)alkyl and beta-(phosphatoxy)alkyl radicals and related species radical and radical ionic migrations and fragmentations of carbon-oxygen bonds, Chem. Rev., 97, 3273, 1997. 

The extent of decarboxylation primarily depends on temperature, pressure, and the stabihty of the incipient R- radical. The more stable the R- radical, the faster and more extensive the decarboxylation. With many diacyl peroxides, decarboxylation and oxygen—oxygen bond scission occur simultaneously in the transition state. Acyloxy radicals are known to form initially only from diacetyl peroxide and from dibenzoyl peroxides (because of the relative instabihties of the corresponding methyl and phenyl radicals formed upon decarboxylation). Diacyl peroxides derived from non-a-branched carboxyhc acids, eg, dilauroyl peroxide, may also initially form acyloxy radical pairs however, these acyloxy radicals decarboxylate very rapidly and the initiating radicals are expected to be alkyl radicals. Diacyl peroxides are also susceptible to induced decompositions ... 

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